A galvanic isolator allows an information signal to pass from its input to its output but has no electrical conduction path between its input and its output. The lack of an electrical conduction path allows the galvanic isolator to prevent unwanted voltages from passing between its input and its output. Strictly speaking, a galvanic isolator blocks only DC voltage, but a typical galvanic isolator additionally blocks a.c. voltage, such as voltages at power line and audio frequencies. An example of a galvanic isolator is a data coupler that passes a high data rate digital information signal but blocks DC voltages and additionally blocks low-frequency a.c. voltages.
One example of a data coupler is an opto-isolator such as the opto-isolators sold by Agilent Technologies, Inc. In an opto-isolator, an electrical information signal is converted to a light signal by a light-emitting diode (LED). The light signal passes through an electrically non-conducting light-transmitting medium, typically an air gap or an optical waveguide, and is received by a photodetector. The photodetector converts the light signal back to an electrical signal. Galvanic isolation is provided because the light signal can pass through the electrically non-conducting light-transmitting medium without the need of metallic conductors.
Other data couplers include a transformer composed of a first coil magnetically coupled to a second coil. Passing the electrical information signal through the first coil converts the electrical information signal to magnetic flux. The magnetic flux passes through air or an electrically non-conducting permeable magnetic material to the second coil. The second coil converts the magnetic flux back to an electrical signal. The transformer allows the high data rate information signal to pass but blocks transmission of DC voltages and low-frequency a.c. voltages. The resistance of the conveyor of the magnetic flux is sufficient to prevent DC voltages and low-frequency a.c. voltages from passing from input to output. Blocking capacitors are sometimes used to provide similar isolation.
Inexpensive opto-isolators are typically limited to data rates of about 10 Mb/s by device capacitance, and from power limitations of the optical devices. The transformer approach requires that the coils have a large inductance yet be capable of transmitting the high data rate information signal. Such conflicting requirements are often difficult to reconcile. Using capacitors does not provide an absolute break in the conduction path because the information signal is transmitted electrically throughout. More successful solutions convert the electrical information signal to another form of signal, e.g., light or a magnetic flux, and then convert the other form of signal back to an electrical signal. This allows the electrical path between input and output to be eliminated.
Many data transmission systems operate at speeds of 100 Mb/s. What is needed is a compact, inexpensive galvanic isolator capable of operating at speeds of 100 Mb/s and above. What is also needed is a compact, inexpensive galvanic isolator that is simple to fabricate and has good common-mode rejection.